ML20135G741
| ML20135G741 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1993-007-00 | |
| Download: ML20135G741 (5) | |
Text
A.6-1 A. 6 LER No. 316/93-007 Event
Description:
Reactor Trip with Degraded Auxiliary Feedwater Date of Event: August 2, 1993 Plant: Cook 2 A.6. 1 Summary Cook 2 tripped from 70% power because of a spurious high-temperature signal from the main turbine exhaust hood. The auxiliary feedwater (AFW) control valves for the east motor-driven AFW pump throttled further than expected, requiring operator action to restore proper flow from that pump. Operator action was also required to reopen two main steam isolation valves (MSIVs) that started drifting closed following the 6
trip. The conditional core damage probability estimated for the event is 2.4 x 10- . The relative significance of this event compared to other postulated events at Cook 2 is shown in Fig. A. 6. 1.
LER 316/93-007 1E-7 I TI 1E-6 1E-5
'LL I E-4 1E-3 II 1E-2 LOF& L 360 h EP L TRI I1MTRAFW LOOP
--- Precursor Cutoff -~360 h AFW Fig. A. 6. 1 Relative event significance of LER 316/93-007 compared with other potential events at Cook 2 A. 6.2 Event Description On August 2, 1993, Cook 2 tripped from 70% power following a turbine trip caused by spurious actuation of the high-temperature switches on the turbine exhaust hood. Eight of the nine switch actuation set points were found to be significantly lower than the as-left condition recorded during the last calibration one year earlier.
Following the reactor trip, the AFW pumps started and provided flow to the steam generators. The feedwater control valves from the east motor-driven AFW pump throttled further than expected after receiving a flow retention signal, and operator action was required to maintain correct flow rates. AFW flow switches were subsequently recalibrated, and flow retention valve intermediate positions were reset to correct the problem.
Two MSIVs, which started to drift closed following the reactor trip, were reopened by the operators. The licensee stated that this drift was expected following a trip because of the valve actuator design.
A. 6.3 Additional Event-Related Information Cook 2 has three AFW pumps; two are motor-driven, and one is turbine-driven. The turbine-driven pump provides flow to all four SGs, and each motor-driven pump provides flow to two SGs. Flow retention valves control the flow from each pump to each SG; these valves can be controlled from the control room. A cross LER No. 3 16/93-007
A.6-2 connect exists that can provide flow from one motor-driven pump to the other unit. The cross-connect valves are manual and normally locked closed. The main feedwater (MFW) pumps are turbine-driven.
A. 6.4 Modeling Assumptions This event was modeled as a reactor trip with degraded AFW and MFW To reflect the reduced flow from the east motor-driven AFW pump, one of the three AFW trains was assumed to be failed in the analysis.
Consistent with other precursor analyses, the probability of not recovering the potentially failed AFW system was not revised because failures were not observed in the other two trains. The accident sequence precursor (ASP) model for MFW assumes that MFW is isolated following a trip but is potentially available in the event of a failure of AFW If all four MSIVs had drifted closed and the operators had not promptly responded and reopened the MSIVs, steam to the MFW pump turbines would have been lost, rendering MFW unavailable. Because this recovery action could be performed in the control room and the observed MSIV response was apparently not unusual at Cook, a nonrecovery probability of 0. 006 was added to the nominal MFW nonrecovery probability (0. 07) to estimate the overall MFW nonrecovery used in the analysis (0. 076).
This probability considered the potential drift of the third and fourth MSIVs (p = 0. 3 and 0. 5, respectively) and the probability that the operators would fail to reopen the drifting-closed MSIVs (p = 0.04). The nonrecovery probabilities used in ASP analyses are described in Sect. A. 3.2 of NUREG/CR-4674, Vol. 17, Precursors to PotentialSevere Core Damage Accidents: 1992, A Status Report.
The potential use of the locked-closed cross-connect between both units' AFW systems was not addressed in the analysis.
A. 6.5 Analysis Results The conditional core damage probability estimated for this event is 2.4 x 106 . The dominant core damage sequences, highlighted on the event tree in Fig. A. 6.2, involves a postulated failure of AFW and MFW following the trip and subsequent failure of feed-and-bleed cooling.
LER No. 316/93-007
A.6-3 PR/ PORV/
PORV SEQ END TRANS RT AFW MFW jSRV SRV HPI HPR OPEN NO STATE CHAL RESEAT OK 01x 11 CO 12 CD OK OK OK 13 CD 14 CD OK OK 15 CD 16 CD 17 CD 18 ATWS Fig. A.6.2 Dominant core damage sequence for LER 316/93-007 LER No. 3 16/93-007
A.6-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event identifier: 316/93-007 Event
Description:
Reactor trip with degraded AFW Event Date: August 2, 1993 Plant: Cook 2 INITIATING EVENT NONRECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OE+OO SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probabili ty CD TRANS 2.4E-06 Total 2.4E-06 ATWS TRANS 3.4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
17 trans -rt AFW MFW hpi(f/b) CD 1.1E-06 1.7E-02 15 trans -rt AFW MFW -hpi(f/b) -hpr/-hpi porv.open CD 1 .1E-06 2.OE-02 16 trans -rt AFW MFW -hpi(flb) hpr/-hpi CD 1.2E-07 2.OE-02 18 trans rt ATWS 3.4E-05 1.2E-01
- nonrecovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
15 trans -rt AFW MFW -hpi(f/b) -hpr/-hpi porv.open CD 1.1E-06 2.OE-02 16 trans -rt AFW MFW -hpi(flb) hpr/-hpi CD 1.2E-07 2.OE-02 17 trans -rt AFW MFW hpi(f/b) CD 1.1E-06 1J.E-02 18 trans rt ATWS 3.4E-05 1.2E-01
- nonrecovery credit for edited case SEQUENCE MODEL: s: \asp\prog\mode Is\pwrbseaLI. cmp BRANCH MODEL: s: \asp\prog\modetIs\cook .s~l PROBABILITY FILE: s:\asp\prog\mode Is\pwr bs 11.pro No Recovery Limit BRANCH FREQUENC IES/PROBAB ILIT IES Branch System Non- Recov Opr Fai L trans 3.4E-04 1.OE+OO loop 1.6E-05 2.4E-01 loca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/ loop 0.OE+OO 1.OE+00 emerg .power 2 .9E -03 8.OE-O1 AFW 3.8E-04 > 5.3E-03 2.6E-01 LER No. 316/93-007
A.6-5 Branch ModeL: 1.OF.3+ser Train 1 Cond Prob: 2.OE-02 > 1.0E+00 Train 2 Cond Prob: 1.OE-01 Train 3 Cond Prob: 5.0E -02 Seriat Component Prob: 2.BE -04 afw/emerg power 5.OE-02 3.4E-01 MFW 1.0E+00 > 1.0E+00 7.OE-02 > 7.6E-02 Branch ModeL: 1.0F.1 Train 1 Cond Prob: 1.OE+00 porv.or.srv.chatL 4.OE-02 1.OE+00 porv.or.srv. reseat 3.OE-02 1.1E-02 porv.or-sr'v. reseat/emerg.power 3.0E-02 1.0E+00 seaLt. oca 2.5E-01 1.0E+00 ep.rec(st) 6.9E-01 1.0E+00 ep. rec 5.2E-02 1.OE+00 hpi 1.OE-03 8. E-01 hpi(f/b) 1.OE-03 8.4E-01 1 O0E-02 hpr/-hpi 1.5E-04 1.OE+00 1.OE-03 porv. open 1.CE-02 1.0E+00 4.OE-04
- branch modeL fiLe
- forced LER No. 316/93-007